linux-kernel - Re: Adding __popcountsi2 and _

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Message-ID: <alpine.DEB.2.21.2505052357091.31828@angie.orcam.me.uk>
Date: Tue, 6 May 2025 00:13:13 +0100 (BST)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
cc: Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev, 
    linux-kernel@...r.kernel.org
Subject: Re: Adding __popcountsi2 and __popcountdi2

On Mon, 5 May 2025, Linus Torvalds wrote:

> > > Once you have the overhead of a function call - and all the register
> > > games etc that involves, you're almost certainly better off with the
> > > simple unconditional bitwise games.
> >
> >  Unless optimising for size, which we do support.
> 
> I think you missed the part where I said
> 
>  "But if you want to do this, put the damn thing as an alias on the code
>   that actually *does* the SW fallback in lib/hweight.c."
> 
> IOW, what I object to is being *stupid* and doing *two* function
> calls. It sure as hell doesn't optimize for size.

 Sure, I only commented on whether or not to have entry points defined in 
the link for libcalls that GCC emits whenever it produces a `popcount' 
operation it cannot find a machine description pattern for (IOW a machine 
instruction or a favourable sequence of).

 They need to have specific names a per the subject and it's up to us to 
implement them one way or another should we choose to; and as I say it 
seems not unreasonable to me for GCC to emit these libcalls at least when 
optimising for size.

 Traditionally we have chosen not to implement double-precision division 
libcalls so as to trap expensive usage.  For `popcount' I have no opinion 
offhand.

  Maciej